Continuous recording viscometer

ABSTRACT

Disclosed is a system for automatically and continuously measuring and producing a continuous record of apparent viscosity, plastic viscosity and yield point for mud systems during a well drilling operation. Also, a continuous record of the ratio of plastic viscosity to yield point values is obtained by the system. Two viscosity determining units are synchronized relative to one another so that their outputs may be used and combined to obtain the above mentioned parameters. Each unit is comprised of a type of Fann viscometer having a sampling cup with a bottom inlet and an open top where overflow may occur. The speed of rotating cylinders in the respective sampling cups, synchronized relative to one another so that when one viscometer operates at 300 rpm the other viscometer operates at 600 rpm, and the outputs of the viscometers can be electronically combined to yield apparent viscosity, plastic viscosity and yield point, as well as the plastic viscosity-yield point ratio.

United States Patent [191 Weiss Dec. 11, 1973 CONTINUOUS RECORDING VISCOMETER [75] Inventor: Walter J. Weiss, Sugarland, Tex.

[73] Assignee: Texaco Inc., New York, NY. [22] Filed: Dec. 20, 1971 [211 Appl. No.: 209,874

[52] US. Cl. 73/59 [51] Int. Cl. G01n 11/10 [58] Field of Search 73/59, 60, 54, 61.4

[56] References Cited UNITED STATES PATENTS 3,069,900 12/1962 Kimberly 73/59 2,679,750 6/1954 Brookfield 73/59 2,382,979 8/1945 Demb 73/59 7 Primary Examiner-Richard C. Queisser Assistant Examiner-Joseph W. Roskos Attorney-Thomas H. Whaley et al.

[57] ABSTRACT Disclosed is a system for automatically and continuously measuring and producing a continuous record of apparent viscosity, plastic viscosity and yield point for mud systems during a well drilling operation. Also, a continuous record of the ratio of plastic viscosity to yield point values is obtained by the system. Two viscosity determining units are synchronized relative to one another so that their outputs may be used and combined to obtain the above mentioned parameters. Each unit is comprised of a type of Fann viscometer having a sampling cup with a bottom inlet and an open top where overflow may occur. The speed of rotating cylinders in the respective sampling cups, synchronized relative to one another so that when one viscometer operates at 300 rpm the other viscometer operates at 600 rpm, and the outputs of the viscometers can be electronically combined to yield apparent viscosity, plastic viscosity and yield point, as well as the plastic viscosity-yield point ratio.

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CONTINUOUS RECORDING VISCOMETER This invention relates to well drilling methods and, more particularly, to well drilling methods for continuously monitoring selected properties of drilling. muds during the drilling operation.

In the drilling of oil br gas wells, one of the most sig nificant factors to control is the properties of the drilling mud system. Among the many uses for drilling mud are the following: use as a lubricant for the drill bit; use for carrying cuttings to the surface; use for providing a mud cake on permeable wall sections; and use for providing sufficient pressure to prevent a blowout. The mud is a mixture of a liquid, usually water, and such additives as are desired for proper control of the well. The additives include solids which are suspended in the liquid.

Because of the character of muds, their flow properties are governed by the laws of plastic flow. As such, properties of drilling mud which are significant include its apparent viscosity,'plastic viscosity and yield point. In present practice, the foregoing properties are usually determined by the operator, from time to time during the drilling operation, by taking a sample from the mud system and testing the sample in a viscometer or with a Marsh funnel. One type of viscometer typically used for this purpose is known as: a Fann viscometer.

A Fann viscometer includes a sampling cup for the mud sample, a rotating sleeve disposed within the cup, and a measuring bob disposed within the sleeve. By relative dimensions of the units and a standardized rpm, relative values of apparent viscosity, plastic viscosity and yield point of the mud can be determined by calibration of adial attached to the bob. Typically, to determine the properties of a mud sample, about -25 minutes are required .while the drilling operation continues. Part of this time elapses because separate tests at different rpms are required, and the operational procedures that are followed for the thixotropic properties typically require the fluid to be sheared at the test rate sufficiently long for the thixotropic forces to reach equilibrium.

The need for measurement and the techniques for measurement as above outlined are discussed in detail in Chapters 2 and 6 of the text Composition and Properties of Oil Well Drilling Fluids by Walter F. Rogers (Gulf Publishing Co., 1963). As disclosed in the Rogers text, viscometer measiirements for the mud sample are obtained at speeds of 300 rpm and 600 rpm of a Fann viscometer. The plastic viscosity u in centiposes, is equal to the difference in measurements for the 600 and 300 rpm speeds so that The yield point V is equal to the 300 rpm measurement less the plasticvelocity, or

Finally, the apparent viscosity p. is equal to one-half of the 600 rpm measurement, or

While it has heretofore been suggested to determine shear stress of a fracturing fluid by simultaneously measuring stress at two different viscometer speeds, there has been no way to derive rheological parameters of the mud system directly and continuously while drilling.

In the drilling of a well, the depth at which the drill is operating is known and the time for mud to reach the bottom of the hole and return can: be calculated. The mud on its return to the surface carries along with it formation cuttings. Also, gas or hydrocarbons may be in the return mud stream and thus affect the weight of the mud. By continuous monitoring of the mud system, various properties can be continuously derived and provide indications of the mud properties as well as any deviations due to gas or hydrocarbons entering into the mud stream.

The present invention contemplates use of two Fanntype viscometerswhich respectively are synchronously driven at sleeve speeds of 300 and 600 rpm. The output from these viscometers is fed through an appropriate electrical network to obtain simultaneous indications of apparent viscosity, plastic viscosity and yield point on a multi-pen recorder, as well as the ratio of plastic viscosity to the yield point. The sampling is continuous in that a mud sample is supplied continuously to the cups through a flow system which includes a lowcapacity relatively surgeless screw pump such as a Moyno. The overflow discharge from the cups is returned to the sampling pit. The sample cups are of sufficient size to allow adequate circulation and yet not too large to permit the mud to channel or become stagnant. The rotating sleeve segment of the measuring units has a cut-away portion to allow free overflow and discharge of test portions of mud.

A Bingharn model for predicting flow characteristics of mud is based upon a straight line relationship between the shear stress and shear rate, with a finite shear stress intercept of 0 shear rate. The foregoing system applies to the Bingham'prediction model. Also employed in the analysis of mud properties is a prediction basis known as a power law model. This model predicts afinite shear stress intercept at 0" shear facets: has a non l irieai'relatibnshipbetween shear stress and shear rate. This non-linear relationship, however, is smooth and predictable, and thus, when viscometers are employed with ranges of, say, 50, 100, 2 00, 300 and 600 rpm, the relationship can be rea sonably defined in the same manner as the Bingham model.

It is an object of the present invention to provide a continuous sampling and recording of drilling mud fluids during a drilling operation. This object and other advantages of the present invention will become apparent from the following discussion when taken in connection with the accompanying drawings in which:

FIG. 1 illustrates a schematic representation of apparatus embodying the present invention relative to a well drilling operation;

FIG. 2 is a more detailed schematic representation of apparatus embodying the present invention;

FIG. 3 is a cross-sectional representation of a portion of the apparatus of the present invention;

FIG. 4 is a view taken along line 4-4 of FIG. 3, and

FIG. 5 illustrates electrical circuitry for deriving the measurements of the present invention.

Apparatus to carry out the present invention is illustrated in the drawings. Referring now to FIG. 1, a borehole is illustrated as extending through various earth formations and is formed by means of a drill bit 1 l. The drill bit 11 is at the lower end of a drilling string 13 which consists of sections of drilling pipe coupled to one another. The upper section of the borehole 10 contains surface casing 14 which is cemented in place. In the upper section of the borehole 10, the drilling string passes through the surface casing 14 and a drilling head (not shown) which is located above a mud return line 15. The mud return line 15 is coupled to the surface pipe 14 to convey the returning mud to the shaker 23 and mud pits 24. 1

Located above the surface pipe 14 is a rotary table drive which is schematically illustrated at 16. The drive 16 is for the purpose of rotating the drilling string and drilling bit in the boirehdle. The drilling string at its upper end is attached to a swivel joint 17 which, in turn, is supported by a hook 18 on a travelling block. The travelling block and hook support the drilling string in a conventional manner. A recorder means 29 can be suitably correlatedl with respect to the travel of the hook 18 by means of a coupling (as schematically illustrated by the dotted line 20) so that the depth of borehole or length of drilling pipe can be readily determined.

An inlet mud pipe 21 is coupled to the swivel head 17 from a mud pump 22 so that mud can be pumped through the interior of the drill string 13 to the bit 11 and, thence, return via the annulus between the wall of the borehole 10 and drilling string 13 to the discharge pipe 15. The return dischhrge pipe 15 is coupled to a shale shaker 23 (shown solhematically) which removes the cuttings from the mud, and the mud is returned to the pit 24. The mud pump ;22 can be supplied with mud for the system from the pit 24.

As shown in FIG. 1, the sampling system includes a sampling pipe 25 which copples the mud return conduit 15 to a selector valve 26;. Another sampling pipe 27 couples the inlet to the pump 22 to the selector valve 26. Dependent upon the position of the selector valve 26, either the pipe 25 or the pipe 27 is coupled to a sampling pump 28. The mud sample from a pipe 25 would represent the mud condition as it returns from the wellbore, while the sample from pipe 27 would represent the condition of mlud as it is input to the well bore.

The pump 28 provides a slow flow of mud to an output conduit 29 which is split into two flow channels 29a and 29b. The flow channels 29a and 29b are respectively coupled to the bases of sampling cups 30 and 40.

The rate of flow relative to the cup size should be such that substantial equilibrium of the thixotropic forces is obtained. The conduit system provides a sample from a common source to the cups at common times of occurrence.

Cups 30 and 40 are identical in configuration and are disposed within a tank 50 which has a return conduit 51 to the mud pit. The cups 30 and 40 respectively are part of separate Fann-type viscometer systems which are coupled to a recorder 52. In the recorder 52 is a chart or film 53 which is driven as a function of time and continuously receives viscosity information from the viscometers. If desired, the recorder 52 can be driven as a function of the depth of the drilling bit or as a combination of time and depth. Correlation of these functions of a well drilling operation to the samples thus can be in accord with the preference of the operator. As will hereafter be more fully explained, the relationship between the viscometers is such that apparent viscosity, plastic viscosity, yield point, and'the plastic viscosity to yield point ratio of the mud are continuously obtained and recorded as a function of the well operation.

Referring now to FIG. 2, a pair of viscometers 31 and 41 are schematically illustrated. These viscometers are generally of the Fann type and include sampling cups 30 and 40, rotating cylinders 34 and 44 and torsion bobs 33 and 43. The rotating cylinders include cylinder measuring portions 34 and 44 disposed within the sampling cups and below the upper levels 32 and 42 of the cups. The driving portion 36 of cylinder 32 is coupled to a gear 37, while the driving portion 46 of cylinder 42 is coupled to a gear 47. The gears 37 and 47 are driven by a common driving gear 38 coupled to a motor 39. The relationship between the gear drives 37 and 47 is 2:1 so that when the viscometer cylinder 32 is driven at 600 rpm, the viscometer cylinder 42 is driven at 300 rpm.

Between therotating lower cylinder measuring portions 34 and 44 for each viscometer and the driving portions 36 and 46 are window sections 35 and 45 which are open below the upper levels 32 and 42 of the cups, thereby permitting flow therethrough. The flow through a cup is between a measuring section and a tor sion bob. The precise geometry and design of the mode of attachment between the portions of the sleeve 34 and 36 or 44 and 46 can take various forms which will be readily apparent to one skilled in the art. The win dows should not impede the flow through the measuring section and should be sufficiently above the test section so as not to affect the validity of the measurements.

At the bottom of the cylindrical part of each sampling cup is a perforated plate 54 to which is attached a centrally located, depending dispersion baffle 55. The dispersion baffle 55 is located within a frustro-conically shaped bottom portion 56 is a sampling cup, where the apices of the bottom portions 56 are coupled to the flow lines 29a and 29b which, in turn, are coupled to the pump 28. Each of the sampling cups 30 and 40 is disposed within a suitable tank 50 so that the overflow from the cups may be returned to the mud system or otherwise be suitably disposed of. The pump 28 is coupled to the valve 26, which can respectively couple the pump to sample either the mud return or the pump intake.

In FIGS. 3 and 4, further details of a typical sampling system for use in the present invention are illustrated. In FIG. 3, the cup 30 has a cylindrical outer wall section 58 which terminates at its upper end with an upper lip 59 and has a frustro-conical lower wall section 56. At the apex of the bottom wall section 56 is an inlet pipe 29a. At the base of the cylindrical section 58 is an internal plate 54 disposed normal to the axis of the cup 30 and which has perforations 60 extending therethrough. Centrally of plate 54 is a depending pin 55 which has a downwardly facing frustro-conically shaped baffle part 61. Baffle 61 is disposed centrally of the inlet 29a and serves to disperse the entering flow of mud and eliminate possible channeling effects. While only structure relative to cup 30 is illustrated, the arrangement relative to cup 40 is of like construction.

In the system thus far described, continuous samples are input to the cup 30 by use of a low-capacity, relatively surgeless screw pump 28 such as a Moyno type. The cup 30 is provided with sufficient size to allow adequate circulation without permitting channeling or permitting the mud to become stagnant. The perforated plate 54 permits a more uniform flow throughout the test section. I

As illustrated in FIGS. 3 and 4, the rotating cylinder portion 34 has a solid wall surface disposed coextensively with the bob 33 and extends above and below the termination of bob. The cylinder portion 34 is disposed intermediate of the upper lip 59 and bottom plate 54 of the cup. Within the cylinder portion 34 is the cylindrical concentrically disposed bob 33 which is attached to an indicating shaft 62. Above the bob 33, the cylinder portion 34has windows 35a, 35b and 350 which are formed by three equidistantly spaced webs. The lower edge of the windows, as noted before, must be disposed a sufficient, distance above bob 33 so as not to affect the integrity of the measurement. The edges of the connecting webs between openings can be curved to avoid a build-up of mud incrustation.

In the operation of a standard Fann viscometer, a sample cup, a rotating cylinder, and a bob as described above are employed. The motor is driven at 300 or 600 rpm, and the force on the bob is obtained from a dial indicator. For a given sample, the dial, which is calibrated in centiposes,.will provide the plastic viscosity value by subtraction of the 300 rpm reading from the 600 rpm reading. The yield point is determined by subtracting the plastic viscosity value from the 300 rpm value. The apparent viscosity is determined by taking one-half of the value from the 600 rpm dial. These measurements can similarly be obtained from the present apparatus as disclosed. The measurements moreover can be automatically and continuously derived.

Referring now to FIG. 5, electrical outputs are ob tained from the torsion applied to the bobs 33 and 34 of the viscometers during the sampling. The electrical outputs are provided as direct current signals from transducers 70 and 71, where transducer 70 provides a D.C. output from the 600 rpm unit and transducer 71 provides a D.C. output from the 300 rpm unit. In general, the voltage output from the 600 rpm unit will be greater than that from the 300 rpm unit. Transducer 70 has output leads 72 and 73, while transducer 71 has output leads 74 and 75. Leads 72 and 74 comprise the positive or high voltage outputs of transducers 70 and 71. Leads 73 and 75 are connected to one another in common. Resistors 76 and 77 are of equal value R and are connected in series between leads 72 and 73. A resistor 78 is connected between leads 74 and 75. Resistor 78 has a value 2R which is twice the resistance value R of either of the resistors 76 or 77. The basic value R of resistors 76 or 77 should be at least ten times greater than the output impedance value of the trans ducers. This prevents loading problems from affecting the voltage measurements or the linearity of the transducers. 1

To obtain a value for plastic viscosity, potential difference between the transducers is used. This is obtainedby taking the output to the first galvanometer from leads 72 and 74. The potential across leads 72 and 74 is equal to the voltage difference between the 600 rpm unit and the 300 rpm unit, and this voltage difference is proportional to plastic viscosity. Thus the galvanometer labelled Galvo l in FIG. 5 will record plastic viscosity on the recorder.

Since the values of resistors 76 and 77 are equal and the voltage drop across the two resistances is equal to the measurement of the viscometer at 600 rpm, the resistors comprise a voltage divider network and the potential between the lead from transducer 71 and a point 79 between the resistors 76 and 77 is equal to one-half the voltage of the 600 rpm measurement. By definition, one-half of the 600 rpm measurement is equal to the apparent viscosity n Thus, by connecting the point 79 and common leads 7.3 and 75 to a galvanometer in the recorder, a record of apparent viscosity can be obtained continuously as would be indicated by the galvanometer labelled Galvo 3 in FIG. 5.

It is known that the yield point is equal to the 300 rpm measurement less the value for the plastic viscosity. Thus, the yield point may be obtained by deriving the voltage potential between the circuit point 79, lead 74 and doubling the measurement. This potential is coupled to a galvanometer (labelled Galvo 2 in FIG. 5) in the recorder.

The measurement signals for the plastic viscosity for Galvo 1 and the measurement signals for the yield point for Galvo 2 are supplied to a ratio circuit 80 which derives a ratio of the plastic viscosity to the yield point. This ratio signal is supplied to a galvanometer in the recorder (labelled Galvo 4 in FIG. 5). By observing the ratio directly, it is possible to determine when flocculation occurs in the system. Flocculation, i.e., the sticking together of particles, and the relative degree of dispersion thus can be quantitatively measured.

As shown in FIG. 1, the recorder 52 can have a clock drive with an indicating mechanism so that a film or paper strip 53 is moved as a function of time where the clock drive produces timing marks or time representations on the log. Thus, each of the measurement traces for apparent viscosity, yield point and plastic viscosity are recorded as a function of time.

While not shown, it is intended that means be provided to flush the system from time to time with clean water without dilutent effects on the mud system.

In the operation of the invention, the recorder is started so that a log with timing marks is initiated. The control valves (not shown) are operated to permit the diversion of mud to the pump 22 from either the ditch or the pit, depending upon the setting of the selector valve 26. The pump provides a flow of mud to the sampling cups. The flow of mud is controlled to allow adequate time for obtaining a reliable indication from the transducers 70 and 71 which automatically is converted by the electrical circuit into appropriate values of apparent viscosity, yeild point and plastic viscosity, which values are recorded on the recorder continuously.

While particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made without departing from this invention in its broader aspects; and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

What is claimed is:

l. A testing system for determining selected properties of a well drilling mud during a drilling operation comprising:

at least two rotational viscometers, each of which has a sampling cup having a cylindrical configuration and a frusto conical bottom portion, perforated means for dispersing mud flow disposed in said cups just above said bottom portion, said means for dispersing each having a depending baffle and said bottom portion each having an inlet opening at its apex, said baffle being disposed in alignment with said opening; means for continuously furnishing to each of said sampling cups a flow of mud which originates from a common source;

means in said viscometers for detecting the shear stress of said mud at different rotational speeds and for deriving functionally related electrical signals;

means for combining said electrical signals in a predetermined relationship for deriving at least one viscosity property of said mud; and

means for continuously recording any such viscosity property as a function of the well drilling operation.

2. The apparatus of claim 1 wherein said detecting means includes an inner torsionally reactant bob and outer rotatable, hollow cylinders having ports for permitting flow of fluid therethrough.

3. The apparatus of claim 1 wherein said combining means includes first resistor means connected across an output of the viscometer operated at the lower speed, second and third equal value resistors connected in series across the output of the viscometer operated at the higher speed, said first resistor having a resistance value of twice the resistance value of either of said second and third resistors, and means for deriving a first electrical voltage signal proportional to the potential difference between the outputs of said first and second viscometers.

4. The apparatus of claim 3 and further including means for deriving a second electrical voltage signal proportional to the potential difference across one of said second and third resistor means.

5. The apparatus of claim 4 and further including means for deriving a third electrical voltage signal proportional to the potential difference between the voltage across said first resistor and said second electrical voltage signal.

6. The apparatus of claim 1 wherein electrical signals derived by said combining means are representative of plastic viscosity and yield point of such mud, and said combining means further includes ration means responsive to any such electrical signals representative of plastic viscosity and yield point for providing a ratio signal to said recording means, said ratio signal being representative of a flocculation factor. 

1. A testing system for determining selected properties of a well drilling mud during a drilling operation comprising: at least two rotational viscometers, each of which has a sampling cup having a cylindrical configuration and a frusto conical bottom portion, perforated means for dispersing mud flow disposed in said cups just above said bottom portion, said means for dispersing each having a depending baffle and said bottom portion each having an inlet opening at its apex, said baffle being disposed in alignment with said opening; means for continuously furnishing to each of said sampling cups a flow of mud which originates from a common source; means in said viscometers for detecting the shear stress of said mud at different rotational speeds and for deriving functionally related electrical signals; means for combining said electrical signals in a predetermined relationship for deriving at least one viscosity property of said mud; and means for continuously recording any such viscosity property as a function of the well drilling operation.
 2. The apparatus of claim 1 wherein said detecting means includes an inner torsionally reactant bob and outer rotatable, hollow cylinders having ports for permitting flow of fluid therethrough.
 3. The apparatus of claim 1 wherein said combining means includes first resistor means connected across an output of the viscometer operated at the lower speed, second and third equal value resistors connected in series across the output of the viscometer operated at the higher speed, said first resistor having a resistance value of twice the resistance value of either of said second and third resistors, and means for deriving a first electrical voltage signal proportional to the potential difference between the outputs of said first and second viscometers.
 4. The apparatus of claim 3 and further including means for deriving a second electrical voltage signal proportional to the potential difference across one of said second and third resistor means.
 5. The apparatus of claim 4 and further including means for Deriving a third electrical voltage signal proportional to the potential difference between the voltage across said first resistor and said second electrical voltage signal.
 6. The apparatus of claim 1 wherein electrical signals derived by said combining means are representative of plastic viscosity and yield point of such mud, and said combining means further includes ration means responsive to any such electrical signals representative of plastic viscosity and yield point for providing a ratio signal to said recording means, said ratio signal being representative of a flocculation factor. 